In recent years, the issues of environment protection are mushrooming. People are endeavoring to achieve the goal of carbon-saving. Meanwhile, the electronic industry is dedicated to develop green products, such as solar cells or light-emitting diodes. To attain the goal of energy saving, the light-emitting diodes have been universally used in a variety of lighting devices, such as household illuminating devices, car illuminating devices, handheld illuminating devices, and electronic billboards.
Referring to FIG. 1, the configuration of the driving device for driving lighting fixture according to the prior art is shown. In FIG. 1, each lighting fixture 1A-1C is placed at different locations in the house according to user's demands. A driving device 1 is used to simultaneously drive lighting fixtures 1A-1C. The driving device 1 is implemented by a two-stage power converter, which includes a first-stage circuit 11 and a second-stage circuit 12. The first-stage circuit 11 is an AC-DC converter and includes a bus capacitor Cbus. The first-stage circuit 11 is used to convert the input voltage Vin, which is in turn filtered and stabilized by the bus capacitor Cbus connected to the output end of the first-stage circuit 11. Therefore, a bus voltage Vbus with a fixed voltage value is generated. The second-stage circuit 12 is connected to the first-stage circuit 11, and is respectively connected to lighting fixtures 1A-1C through a first lighting fixture base 131A, 131B, a second lighting fixture base 132A, 132B, and a third lighting fixture base 133A, 133B. The second-stage circuit 12 includes several converters (not shown), in which the number of the converters of the second-stage circuit 12 is identical to the number of the lighting fixtures 1A-1C. Thus, the converters of the second-stage circuit 12 can convert the bus voltage Vbus into operating voltages Vo1, Vo2, Vo3, respectively. The operating voltages Vo1, Vo2, Vo3 are respectively supplied to the lighting fixtures 1A-1C through the first lighting fixture base 131A, 131B, the second lighting fixture base 132A, 132B, and the third lighting fixture base 133A, 133B.
When the lighting fixture switch 10 is turned on, the input voltage will be supplied to the input end of the first-stage circuit 11 through the lighting fixture switch 10 and converted into a bus voltage Vbus with a voltage value of 250V-400V by the first-stage circuit 11. The bus voltage Vbus is supplied to the second-stage circuit 12 and is reduced to output a first operating voltage Vo1, a second operating voltage Vo2, and a third operating voltage Vo3 by the second-stage circuit 12, thereby driving the lighting fixtures 1A-1C to illuminate.
As each circuit stage has power loss, the electric energy delivered to the lighting fixture will be reduced as a result of the conversion carried out by the first-stage circuit 11 and the second-stage circuit 12. This would deteriorate the conversion efficiency and waste electric energy. More disadvantageously, the operating efficiency of the driving device 1 can not be enhanced, which would further cause the goal of carbon-saving to be unattainable.
In addition, the driving device 1 is configured to drive LED lighting fixtures of the same specification. As the LED is an element with its luminance being positive proportional to the current, the second-stage circuit 12 has to supply operating currents Io1, Io2, Io3 which have a substantially identical current value with each other to the lighting fixtures 1A-1C, thereby balancing the luminance among the LED lighting fixtures. However, the performances of the converters are inconsistent as the manufacturing of the elements of the converters are different and the elements of the converters have tolerance. Therefore, the current values of the operating currents Io1, Io2, Io3 are not identical with each other.
Furthermore, if the conventional driving device 1 desires to perform dimming operation to the lighting fixtures 1A-1C that are made up of LEDs, each converter of the second-stage circuit 12 requires a dimming circuit and a control unit. As the dimming circuit and the control unit have a complicated circuit structure and a high cost, the manufacturing cost of the conventional driving device 1 for lighting fixture will be boosted and the circuit design of the conventional driving device 1 for lighting fixture will be difficult.
Moreover, the bus capacitor Cbus is used to filter and stabilize the bus voltage Vbus outputted from the first-stage circuit 11. As the voltage value of the bus voltage Vbus is about 250V-400V, the bus capacitor Cbus has to be implemented by a capacitor with high rated capacitance and excellent voltage durability. In this manner, the bus capacitor Cbus will be bulky and expensive. Also, in order to prevent the driving device 1 or the lighting fixture 1A-1C from being short-circuited as a result of damage, the bus capacitor Cbus with a high rated capacitance has to discharge. This would cause a large current surge, which would in turn damage the internal electronic elements of the driving device 1. Also, the driving device 1 requires additional protection circuits to protect the electronic elements. This would further increase the manufacturing cost of the driving device for lighting fixture.
Hence, it is inclined to develop a driving device for light fixture that can address the aforementioned drawbacks and can perform dimming operation to the lighting fixtures. The invention is proposed to meet these requirements.